Display device

ABSTRACT

There is disclosed a display device using a color wheel having a color filter Cw, in addition to normal color filters corresponding to the three primary colors. The filter Cw has almost flat spectral transmission characteristics. Brightness information included in a color image signal is quantized with (n+m) bits. Information corresponding to the lower-order n bits is displayed by light transmitted through the filter Cw. Information corresponding to the upper-order n bits is displayed by light transmitted through the normal color filters. Only brightness information to which the human eye is visually sensitive is reproduced by the filter Cw having a lower transmissivity. This can eliminate a lack of the number of gray levels due to a constraint on the minimum switching speed of a light valve. Furthermore, the brightness little deteriorates.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a display device and, moreparticularly, to a display device using color filters to reproducecolors.

[0003] 2. Description of the Prior Art

[0004] In recent years, numerous display devices have been available inwhich color filters are used to decompose light from a light source intoN colors that are projected onto a screen for reproducing a color image,where N is a positive integer. Normally, N=3, and light is decomposedinto red (R), green (G), and blue (B) colors which are projected toreproduce a color image. The simplest example of implementation forachieving this is given below.

[0005]FIG. 1 shows an example of a display device, comprising a lightsource 101, a color wheel 102, a light valve 103, a screen 104, anddrive electronics 105. The display device shown in FIG. 1 is assumed toproject light decomposed into R, G, and B colors, thus reproducing colorimages.

[0006] The operation of the display device constructed as describedabove is described by referring to FIG. 1. Seven-bit color image datahaving a frame rate of 60 Hz and a synchronizing signal are applied tothe drive electronics 105. The drive electronics 105 create controlsignals for the color wheel 102 and for the light valve 103 from theentered color image data and the synchronizing signal. The controlsignals are fed to the color wheel 102 and to the light valve 103.

[0007] The light valve 103 is a device for turning ON or OFF eachindividual pixel. A digital micromirror device (DMD), a liquid crystal,or the like is used as the light valve 103. Where the DMD is used as thelight valve 103, the direction in which light is reflected is controlledfor each individual pixel, thus turning ON or OFF the light. Where thelight is reflected toward the screen, the device is turned ON. Where thelight is reflected toward the outside the screen, the device is turnedOFF. This is referred to as control of the reflection.

[0008] Where a liquid crystal is used as the light valve 103, thefollowing two types are conceivable. One type controls reflection in thesame way as the aforementioned DMD. Another type switches ON and OFFpassage of light for each individual pixel. Where the light istransmitted, the device is turned ON. Where the light is nottransmitted, the device is turned OFF. The transmitted light is broughtto a focus on the screen.

[0009] An ultrahigh-pressure mercury lamp is used as the light source101, for example. Light emitted from this lamp is made to hit a part ofthe color wheel 102. This color wheel 102 is divided into threesegments, for example. These segments are color filters Cr, Cg, and Cbthat transmit R, G, and B, respectively. The color wheel 102 makes onerevolution in {fraction (1/60)} msec, i.e., about 16.667 msec (3600 rpm). This rotation is synchronized to the frame rate (60 Hz in the aboveexample) of the displayed image.

[0010] Where light from the light source 101 shines on the color filtersegment Cr on the color wheel 102, the light valve 103 is controlled bycolor image data about R. An R image is projected onto the screen 104.With other colors, the light from the light source 101 is similarlyprojected onto the screen 104 via the color filters on the color wheel102 and via the light valve 103, and images are displayed.

[0011] The times for which the light from the light source 101 is madeto shine on the segments of the color wheel 102 during one revolution ofthe color wheel 102 are next described. The light source 101 illuminatesparts of the color wheel 102. The produced light spot has some diameter.Where this light spot is at the boundary between two adjacent colorfilters, two colors across the boundary will be mixed up. That is, onelight spot has two colors of light transmitted through the colorfilters. This cannot be used for image display. Therefore, where thelight spot shines on the boundary, it is necessary to turn OFF the lightvalve.

[0012] For the sake of illustration, it is assumed that the light valvemust be kept OFF within an angular range of 15° on the color wheel 102.Of course, this angular range may differ, depending on the size of thelight spot and on the sizes of the segments forming the color filters.

[0013] As can be seen from FIG. 1, the boundaries between the colorfilters on the color wheel 102 are three boundaries between R, G, and Bcolors. During one revolution of the color wheel 102, it is necessary toturn OFF the light valve 103 for a time corresponding to an angularrange of 15×3=45°. This time is referred to as the ineffective time. Theother time is referred to as the effective time.

[0014] Since the color wheel 102 makes one revolution in about 16.667msec, the ineffective time is 45°/360°×16.667≡approximately 2.083 msec.Of the effective time, the time for which the light shines on the colorfilter Cr is equal to the effective time divided by 3, i.e., about 4.862msec (({fraction (1/60)}××(1- 45°/360°))/3≡16.667−2.083) msec/threecolors. Similarly, the time for which the light shines on the colorfilters Cg and Cb is about 4.862 msec.

[0015] A method of reproducing gray levels is now described by takingthe case of R as an example. When the light shines on the color filterCr during the effective time of the color wheel 102, the light valve 103is controlled according to an R image signal. Where the first gray levelis displayed, the light valve 103 is turned ON for about 0.038 msecwithin the time for which the light shines on the color filter Cr duringone revolution of the color wheel 102. The light valve is kept OFFduring the remaining time of about 4.824 msec. Where the second graylevel is displayed, the light valve 103 is turned ON for twice of the ONtime for the first gray level, i.e., 0.076 msec. The light valve is keptOFF during the remaining time of 4.786 msec. Where the third, fourth, .. . , and 127th gray levels are displayed, the light valve is turned ONfor 3 times, 4 times, . . . , and 127 times, respectively, of the ONtime for the first gray level. The light valve is kept OFF during theremaining times. Thus, there are 128 combinations of ON/OFF timesincluding a fully OFF state.

[0016] The human eye does not respond to flickers higher than 60 Hz,which is generally known as the critical flicker frequency. As the ONtime prolongs within the 16.667 msec, the human eye feels brighter. Asthe ON time shortens, the eye feels darker. The human eye perceives 128ON/OFF time combinations as 128 gray levels. Light is projected onto thescreen such that the light valve is turned ON or OFF for each pixel, andan R image that visually has gray levels is reproduced. With respect toeach of G and B, 128 gray levels are reproduced in the same way as inthe case of R.

[0017] Each image of R, G, and B is projected in turn onto the screenfor one third of 1 frame time of about 16.667 msec, i.e., about 5.556msec. As mentioned above, the human eye does not respond to flickershigher than the critical fusion frequency of 60 Hz and so he or shefeels as if three colors were displayed simultaneously. Consequently, acolor image is visually reproduced.

[0018] In the example given above, gray levels corresponding to 7 bits,i.e., 128 gray levels (2⁷ gray levels), are represented. The light valve103 is switched ON and OFF at intervals of about 0.038 msec, i.e., thetime (about 4.862 msec) for which light is made to shine on the colorfilter Cr divided by 127 (128-1) that is the number of gray levelsexcluding the zeroth gray level at which light is not output.

[0019] Where it is attempted to display a wider range of gray scale withthe above-described structure, e.g., gray levels (2⁸=256 gray levels)corresponding to 8 bits, it is necessary to switch ON and OFF the lightvalve 103 at intervals within the time for which light is made to shineon the color filter Cr divided by 255, i.e., about 0.019 msec, if theprinciple described above is applied.

[0020] Where the light is turned ON and OFF using the light valve 103such as a DMD as mentioned above, however, the minimum switching timeachievable with the presently available DMD is about 0.030 msec.Therefore, it is impossible to switch the device ON and OFF at intervalsof about 0.019 msec as described above.

[0021] Where the light is turned ON and OFF using the light valve 103 asconsisting of a DMD in an attempt to solve the above-described problem,the minimum switching time is about 0.030 msec as described above. Astructure capable of displaying 1024 gray levels (2¹⁰ gray levels) withthis structure is disclosed, for example, in Japanese Unexamined PatentPublication No. 149350/1997.

[0022] This disclosed display device is shown in FIG. 2. Note that likecomponents are indicated by like reference numerals in various figuresand those components which have been already described in connectionwith FIG. 1 will not be described below. A color wheel 202 is dividedinto 6 segments to form color filters Crd, Cgd, and Cbd of lowertransmissivity than color filters Cr, Cg, and Cb, in addition to theconventional filters Cr, Cg, and Cb. The transmissivity of the filtersCrd, Cgd, and Cbd is one eighth of that of the filters Cr, Cg, and Cb.Thus, gray levels corresponding to the 3 bits, i.e., 2³ gray levels (8gray levels), are added.

[0023] The structure shown in FIG. 2 and its operation are nowdescribed. Drive electronics 205 receive 10-bit color image data havinga frame rate of 60 Hz and a synchronizing signal. The drive electronics205 create control signals for a color wheel 202 and for a light valve103 from the input color image data and send these control signals tothe wheel and to the light valve.

[0024] Of the 6 segments on the color wheel 202, the color filters Crand Crd transmit R. The color filters Cg and Cgd transmit G. The colorfilters Cb and Cbd transmit B. The transmissivity of the filter Crd isone eighth of that of the filter Cr. The transmissivity of the colorfilter Cgd is one eighth of that of the filter Cg. The transmissivity ofthe color filter Cbd is one eighth of that of the filter Cb.

[0025] The color wheel 202 makes one revolution in {fraction (1/60)}msec≡16.667 msec. This rotation is synchronized to the frame rate of thedisplayed image. In the structure shown in FIG. 2, there are 6 colorfilters and so there exist 6 boundaries as can be seen from the figure.In this case, therefore, the ineffective time is about 15°×6/360°×16.667msec≡4.167 msec. The effective time is about 16.667 msec−4.167msec=12.500 msec.

[0026] The time for which the light from the light source 101 is made toshine on the color filter Cr of the color wheel 202 during onerevolution of the color wheel 202 is one third of the aforementionedeffective time (12.500 msec) multiplied by a proportion at which lightis made to shine on the color filter Cr, i.e., about 12.500msec/3×127/(127+7)=3.949 msec. The segment of the color filter Cr isdetermined based on this time. Similarly, the time for which light ismade to hit the color filters Cg and Cb is also about 3.949 msec.

[0027] The time assigned to illuminate the color filter Crd is one thirdof the effective time (12.500 msec) multiplied by the proportion atwhich the filter Crd is illuminated, i.e., about 12.500msec/3×7/(127+7)=0.218 msec. The segment of the color filter Crd isdetermined based on this time. Similarly, the time for which the colorfilters Cgd and Cbd are illuminated is about 0.218 msec.

[0028] A method of reproducing gray levels is now described, taking R asan example. The time for which the color filter Cr of the color wheel202 is illuminated is controlled according to R color image data. Wherethe first gray scale of the R image signal represented by the filter Cris displayed, the light valve 103 is turned ON for about 0.031 msec(=3.949 msec/127) of the time for which the filter Cr is illuminatedduring one revolution of the color wheel 202. The valve 103 is kept OFFduring the remaining time.

[0029] Where the second gray level represented by the color filter Cr isdisplayed, the light valve 103 is maintained ON during twice of the ONtime for the first gray level represented by the filter Cr, i.e., about0.062 msec. The valve is kept OFF during the remaining time. Where thethird, the fourth, . . . , and the 127th gray levels are displayed, thelight valve is turned ON for 3 times, 4 times, . . . , 127 times,respectively, of the ON time for the first gray level. The light valveis kept OFF during the remaining times. Thus, there are 128 combinationsof ON/OFF times and thus 128 gray levels can be represented.

[0030] A method of displaying 1024 R gray levels using the color filterCrd is now described. Where the first gray level represented by thefilter Crd is displayed, the light valve 103 is kept ON for about 0.031msec (=0.218 msec/7) within the time for which the filter Crd isilluminated during one revolution of the color wheel 202. The valve iskept OFF during the remaining time. Where the second gray levelrepresented by the filter Crd is displayed, the valve is kept ON fortwice of the ON time for the first gray level represented by the filterCrd, i.e., 0.062 msec. The valve is kept OFF during the remaining time.Where the third, fourth, ..., and 7th gray levels represented by thefilter Crd are displayed, the light valve is kept ON for 3 times, 4times, . . . , 7 times, respectively, of the ON time for the first graylevel represented by the filter Crd. The light valve is kept OFF duringthe remaining time. Thus, there are 8 combinations of ON/OFF timesincluding a fully OFF state and thus 8 gray levels can be represented.

[0031] The transmissivity of the color filter Crd is one eighth of thatof the filter Cr. The brightness of the first gray level displayed usingonly the color filter Crd is one eighth of that of the first gray leveldisplayed using only the filter Cr. That is, using combinations of thecolor filters Cr and Crd, 128 gray levels (provided by the color filterCr)×8 gray levels (provided by the color filter Crd)=1024 gray levels(2¹⁰ gray levels) can be represented.

[0032] Accordingly, of color image data (R image data in this example)quantized with 10 bits (2¹⁰), the upper-order 7 bits are expressed usingthe color filter Cr, while the lower-order 3 bits are expressed usingthe color filter Crd. In this way, 1024 gray levels can be reproduced.

[0033] With respect to G and B, the upper-order 7 bits are expressedusing the color filters Cg and Cb. The lower-order 3 bits arerepresented using the color filters Cgd and Cbd. In this manner, 1024gray levels can be reproduced. Images of R, G, and B are projected ontothe screen 104 by this gray scale control. A color image is perceived bythe human visual characteristics.

[0034] Where 1024 gray levels are expressed using the structure andprocedure described above, the light-transmitting region of the colorwheel 202 is divided into 6 segments corresponding to the differentcolors and different gray levels. Therefore, there are 6 boundariesbetween the color filters. The ineffective time due to the boundaries isdoubled compared with the case in which there are only three boundariesbetween color filters. Finally, the brightness of the image projectedonto the screen is decreased by about 14%.

[0035] In addition to this decrease in the brightness, the presence ofthe color filters Crd, Cgd, and Cbd having a transmissivity that is onlyone eighth of that of the color filters Cr, Cg, and Cb lowers thebrightness.

SUMMARY OF THE INVENTION

[0036] In view of the foregoing problems, the present invention has beenmade.

[0037] It is an object of the present invention to provide a displaydevice which is capable of representing gray levels more than the numberof gray levels limited by the minimum switching time at which a lightvalve is turned ON and OFF and which suffers almost no brightnessdecrease.

[0038] A display device in accordance with the present invention acts todisplay an image according to input image data and comprises a lightsource, light-transmitting filters for separating the light from thelight source into at least four kinds of light including white light,and a light valve for projecting each kind of light transmitted throughthe filters onto a screen.

[0039] Some gray levels have been heretofore impossible to display dueto restrictions imposed by the minimum switching time at which the lightvalve is turned ON and OFF. Information about only visually sensitivebrightness levels is reproduced using the light-transmitting filtercorresponding to white light. Hence, smoother gray-scale representationcan be accomplished without deteriorating the brightness.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a block diagram of the prior art display device;

[0041]FIG. 2 is a block diagram of a known display device disclosed inJapanese Unexamined Patent Publication No. 149350/1997;

[0042]FIG. 3 is a block diagram of a display device in accordance withthe present invention;

[0043]FIG. 4 is a block diagram of a signal converter portion in adisplay device in accordance with the invention;

[0044]FIG. 5 is a graph illustrating a method of displaying gray levelswith a display device in accordance with the invention; and

[0045]FIG. 6 is a graph illustrating another method of displaying graylevels with a display device in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0046] First Embodiment

[0047] Referring to FIG. 3, there is shown a block diagram of a displaydevice in accordance with the present invention. This display devicecomprises a light source 101, a color wheel 2, a light valve 103, ascreen 104, a signal converter portion 6, and drive electronics 5.

[0048] As shown in FIG. 3, the color wheel 2 is divided into 4 segmentsincluding color filters Cr, Cg, and Cb that transmit R, G, and B,respectively. The color wheel 2 further includes a color filter Cw suchas a neutral density filter that transmits white light. This filter Cwshows almost flat spectral characteristics, as opposed to the filtersCr, Cg, and Cb. Let the color filters Cr, Cg, Cb, and Cw havetransmissivities of fr(λ), fg(λ), fb(λ), and fw(λ), respectively. fw(λ)is so set as to satisfy Eq. (1) below.

[0049]   (1)

[0050] where (λ) is the wavelength of light, V(λ) is the relativespectral sensitivity characteristic of the human eye, and 1/K is acoefficient determining the transmissivity of Cw.

[0051] If the coefficient K at the right side of Eq. (1) above is set to8 (K=8), the color filters Cw, Cr, Cg, and Cb assume suchtransmissivities that the brightness of the first gray level representedusing only the color filter Cw is one eighth of the brightness achievedwhen the three filters Cr, Cg, and Cb simultaneously represent theirfirst gray levels. That is, the integrated value of the transmissivityin the visible range (light wavelength λ lies between 380 nm and 780 nm)of the light-transmitting filter (color filter Cw) corresponding towhite light is smaller than the integrated values of thetransmissivities in the visible range of the other light-transmittingfilters Cr, Cg, and Cb.

[0052] As mentioned above, where a light valve such as a DMD is used, ifthe minimum switching time is 0.030 nm, it is difficult to achieve 256gray levels. Therefore, 1/K is set to ½^(p) (where P is a naturalnumber), i.e., ½, ¼, ⅛, {fraction (1/16)}, and so forth. However, whereK has a small value, the minimum switching time of the light valve posesa constraint. Where K has a large value, the segment Cw widens and thusthe color filters Cr, Cg, and Cb become narrowed. This will narrow thefull range of gray scale in representing R, G, andB colors. Of theselimiting conditions, K=8 is selected because it is well applied to adisplay device. This case is discussed below.

[0053] The display device in accordance with the present embodimentconstructed in this way decomposes light into 4 colors by the colorfilters Cr, Cg, Cb, and Cw. The 4 colors of light are projected via thelight valve 103 onto the screen 104, thus reproducing a color image.

[0054] The operation of the display device shown in FIG. 3 is nextdescribed. Color image data of 10 bits having a frame rate of 60 Hz isinput to the signal converter portion 6. This converter portion 6converts the input color image data as follows and sends it to the driveelectronics 5. The drive electronics 5 also receive a synchronizingsignal.

[0055] The manner in which the signal converter portion 6 converts itsinput color image data is now described by referring to FIG. 4, which isa detail block diagram of the signal converter portion 6. This converterportion 6 has input terminals 7, 8, and 9 receiving 10-bit, color imagedata Rin, Gin, and Bin, respectively, corresponding to the R, G, and Bcolors.

[0056] The signal converter portion 6 further includes a brightnesssignal calculating unit 10 for calculating brightness data Y satisfyingEq. (2) below, assuming that the lower-order 3 bits of the input colorimage data Rin, Gin, and Bin are Sr, Sg, and Sb, respectively. Theupper-order 3 bits of the brightness data Y is supplied as a convertedcolor image data Wout to an output terminal 17. Delay compensatingportions 11, 12, and 13 delay the upper-order 7 bits of the signals Rin,Gin, and Bin coming from the input terminals 7, 8, and 9 by an amountequal to the time taken for the signal calculating unit 10 to calculatethe brightness signal. The obtained data are sent as converted imagedata Rout, Gout, and Bout to output terminals 14, 15, and 16, at thetiming of the data Wout.

Y=0.299Sr+0.587Sg+0.114Sb   (2)

[0057] The output terminals 14, 15, 16, and 17 are connected with thedrive electronics 5, which in turn create control signals for the colorwheel 2 and light valve 103 from the converted color image data Rout,Gout, Bout,Wout and from the synchronizing signal and send the controlsignals to the color wheel 2 and to the light valve 103.

[0058] The color wheel 2 makes one revolution in {fraction (1/60)}msec≡16.667 msec (3600 rpm). This rotation is synchronized with theframe rate of the displayed image. The color wheel 2 has 4 color filtersthat form four boundaries as can be seen from the figure. In this case,therefore, the ineffective time is about 15°×4/360°×16.667 msec≡2.778msec. The effective time is about 16.667 msec−2.778 msec=13.889 msec.

[0059] During one revolution of the color wheel 2, the time assigned toilluminate the color filter Cr of the color wheel 2 with the light fromthe light source 101 is 4.546 msec, for the following reason. The threesegments Cr, Cg, and Cb produce 128 gray levels. One segment Cw produces8 gray levels. The effective time of about 13.889 msec corresponds tothese three segments Cr, Cg, Cb and one segment Cw. Since the colorfilter Cr produces 128 gray levels, the ratio of the time assigned tothe color filter Cr to the effective time of about 13.889 msec is foundby calculating (the time for which the color filter Cr is illuminated)divided by (the time for which the color filter Cr is illuminated×thetime for the 3 segments+the time for which the color filter Cw isilluminated). That is the time assigned to the color filter Cr is about13.889 msec ×127/(3×l27+7)=4.546 msec. The segment Cr is determinedbased on this time of 4.546 msec. Similarly, the color filters Cg and Cbare illuminated for 4.546 msec.

[0060] The time assigned to illuminate the color filter Cw is discussed.The effective time of about 13.889 msec corresponds to 3 segments Cr,Cg, and Cb producing 128 gray levels and 1 segment Cw producing 8 graylevels. Since the segment of the color filter Cw produces 8 gray levels,the ratio of the time assigned to the color filter Cw to the effectivetime is found by calculating (the time for which the color filter Cw isilluminated) divided by (the time for which the color filter Cr isilluminated×the time for the 3 segments+the time for which the colorfilter Cw is illuminated) . That is, about 13.889 msec×7/(3×127+7)=0.251msec is the time assigned to the color filter Cw. The segment of thecolor filter Cw is determined based on this time.

[0061] A method of reproducing gray scales of R is now described. Thelight valve 103 is controlled according to the converted color imagedata Rout about R produced from the output terminal 14 of the signalconverter portion 6 while the color filter Cr is being illuminated.Where the first gray level of the converted color image data Rout aboutR is displayed, the light valve 103 is kept ON during about 0.036 msec(4.546 msec/177 gray levels) within the time for which the color filterCr is illuminated during one revolution of the color wheel 2. The valve103 is kept OFF during the remaining time. Where the second gray levelis displayed, the valve 103 is kept ON during twice of the time for thefirst gray level (i.e., 0.072 msec). The valve 103 is kept OFF duringthe remaining time. Where the third, fourth, . . . , and 127th graylevels are displayed using the Rout, the light valve 103 is kept ON for3 times, 4 times, . . , , 127 times, respectively, of the ON time forthe first gray level of the Rout. The valve 103 is kept OFF during theremaining time. Thus, there are 128 combinations of ON/OFF timesincluding a fully OFF state.

[0062] As mentioned above, the human eye does not respond to flickershigher than the critical fusion frequency of 60 Hz and so he or shefeels brighter with increasing the ON time within the period of 16.667msec and feels darker with decreasing the ON time. The human eyeperceives the 128 combinations of ON/OFF times as 128 gray levels.Inexactly the same way, 128 gray levels are reproduced from G and B.

[0063] Now, a method of reproducing gray scales of 129 and more usingthe color filter Cw is described. Of the color image data Rin, Gin, andBin quantized with 10 bits, the upper-order 7 bits are displayed usingthe capability of the color filters Cr, Cg, and Cb to reproduce 128 graylevels. Of the color image data Rin, Gin, and Bin quantized with 10bits, the lower-order 3 bits are displayed as 3-bit color image dataWout (having 2³ gray levels=8 gray levels) using the color filter Cw.

[0064] Where the first gray level of the 3-bit color image data Wout isdisplayed, the light valve 103 is kept ON for 0.036 msec (=0.251 msec/7)within the time for which the color filter Cw is illuminated during onerevolution of the color wheel 2. The valve 103 is kept OFF during theremaining time. Where the second gray level of Wout is displayed, thevalve 103 is kept ON during twice of the ON time for the first graylevel, i.e., 0.072 msec. The valve 103 is kept OFF during the remainingtime. Where the third, fourth, . . . , and seventh gray levels aredisplayed, the light valve 103 is kept ON during three times, fourtimes, . . . , and 7 times, respectively, of the time for the first graylevel represented by Wout. The valve 103 is kept OFF during theremaining time. In this way, 8 gray levels including a fully OFF statecan be represented.

[0065] With respect to the transmissivity of the color filter Cw, it isnow assumed that K=8, which is substituted into Eq. (1). In this case,the brightness of the first gray level of Wout represented using onlythe color filter Cw is one eighth of the brightness obtained where thethree color filters Cr, Cg, and Cb simultaneously provide theirrespective first gray levels. Therefore, where the display image is ablack-and-white image, the upper-order 7 bits of the image dataquantized with 10 bits can represent 2⁷ gray levels=128 gray levelsusing the color filters Cr, Cg, and Cb. The lower-order 3 bits canrepresent 2³ gray levels=8 gray levels using the color filter Cw.Consequently, 1024 gray levels can be represented.

[0066] This is described in detail by referring to FIGS. 5(a)-5(d). FIG.5(a) shows a signal applied to the signal converter portion 6. FIG. 5(b)shows the brightness of an image reproduced by the color filters Cr, Cg,and Cb. FIG. 5(c) shows the brightness of an image reproduced by thecolor filter Cw. FIG. 5(d) shows the brightness of the resultant of theimages shown in FIGS. 5(b) and 5(c) perceived by the human visualcharacteristics. It can be observed that the number of gray levels shownin FIG. 5(d) is the same as the number of gray levels shown in FIG.5(a).

[0067] Where the displayed image is not a black-and-white image but acolor image, the brightness components of the color image data quantizedwith 10 bits can produce 1024 gray levels using the color filters Cr,Cg, Cb, and Cw. However, with respect to color components, only 128 graylevels can be produced using the color filters Cr, Cg, and Cb.Furthermore, the chroma deteriorates slightly, because white and blackcomponents are mixed by the color filter Cw. However, the visualcharacteristics of the human eye have such a feature that the eye candiscriminate a less number of color gray levels than brightness graylevels. Consequently, this will not present great problems in practicalsituations.

[0068] The addition of only the color filter Cw to the three colorfilters Cr, Cg, and Cb described above can increase the number of graylevels of brightness. Therefore, the decrease in the brightness is onlyabout 3%, compared with the instrument comprising the three colorfilters. As a result, the effects of the decrease in the brightnesspresent almost no problems.

[0069] The color filter Cw is only required to exhibit almost flatspectral transmission characteristics in the visible range. This filteris not limited to a filter that transmits pure white light. For example,to adjust the color temperature of the reproduced image, the spectralcharacteristics are allowed to be shifted slightly toward red or blue.

[0070] Second Embodiment

[0071] In the first embodiment, the color filter Cw is used from thefirst to the 1024th gray level. It is not necessary to use the colorfilter Cw for all the gray levels. The filter Cw may be employed onlyfor dark image portions. An example of operation in this case is nextdescribed by referring to FIGS. 6(a)-6(d) FIG. 6(a) shows an imagesignal applied to the signal converter portion 6. FIG. 6(b) shows thebrightness of an image reproduced by the color filters Cr, Cg, and Cb,and is the same as obtained in the first embodiment. FIG. 6(c) shows thebrightness of an image reproduced by the color filter Cw. This filter Cwis used for only the 15th gray level and below. The filter Cw is keptOFF in response to the 16th gray level and above. The resultantbrightness of the color filters Cr, Cg, Cb, and Cw is shown in FIG.6(d).

[0072] The human eye's capability to discriminate bright portions islower than the human eye's capability to discriminate dark portions.Therefore, where the color filter Cw is used only for dark portions toresolve gray levels, the same advantages can be obtained as the firstembodiment. Furthermore, the 16th and higher gray levels can bedisplayed in the same way as the prior art instrument. The decrease inthe chroma due to mixing of white and black components by the colorfilter Cw can be suppressed to a minimum.

[0073] Third Embodiment

[0074] In the first and second embodiments, 10 bits of image data areseparated into the upper-order 7 bits and the lower-order 3 bits anddisplayed. The present invention is not limited to this separationmethod. For example, (n+m)-bit image data (where n and m are anyarbitrary numbers equal to or greater than 0) may be divided into theupper-order n bits and the lower-order m bits and displayed. It is onlynecessary that the upper-order n bits and the lower-order m bitssuitable for the characteristics of the display device be established.

[0075] Fourth Embodiment

[0076] In the first through third embodiments, Eq. (2) is used tocalculate brightness data Y. The invention is not restricted to the useof this equation. Rather, appropriate coefficients may be used accordingto the spectral characteristics of the color filters Cr, Cg, Cb, and Cw.Furthermore, coefficients assigned to the filters Sr, Sg, and Sb may beappropriately varied to reduce the size of the hardware.

[0077] Where signals are transmitted such that a Y signal (luminancesignal) and a chrominance signal are combined as in normal TV, only theY signal may be used, though the chrominance signal is also transmitted.

[0078] Fifth Embodiment

[0079] In the first through fourth embodiments, a color filterexhibiting flat spectral characteristics in the visible range is used asthe color filter Cw. The invention is not limited to the use of thisfilter. Rather, the filter may have any desired characteristics as longas it transmits white light within a realizable range. For instance, thecharacteristic curve may have some peaks and valleys. Filters havingthese characteristics may have advantages similar to those yielded bythe aforementioned filters.

[0080] In the descriptions provided thus far, color filterscorresponding to Y (yellow), M (magenta), and C (cyan) may be formed onthe color wheel, in addition to R, G, and B. The color filters are notalways restricted to R, G, and B color filters.

[0081] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential parts thereof. The aboveembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A display device for displaying an imageaccording to input image data that is digital data, said display devicecomprising: a light source for producing light; light-transmittingfilters for separating the light from said light source into at leastfour kinds of light including white light, said light-transmittingfilters including a white-transmitting filter for transmitting whitelight and non-white transmitting filters; a light valve for projectingeach kind of light from said light-transmitting filters onto a screen;said white light-transmitting filter being used to display informationcorresponding to lower-order bits of said digital data; and saidnon-white light-transmitting filters being used to display informationcorresponding to higher-order bits of said digital data.
 2. The displaydevice of claim 1, wherein said white light-transmitting filter hasspectral characteristics that are almost flat in the visible range ofwavelengths of the light.
 3. The display device of claim 1, wherein if abrightness required by the input image data is lower than a given graylevel, information is displayed using said white light-transmittingfilter or said non-white light-transmitting filters, and if saidbrightness is higher than said given gray level, information isdisplayed using only said non-white light-transmitting filters.
 4. Thedisplay device of claim 1, wherein said light valve is of the reflectivetype.
 5. The display device of claim 1, wherein said light valve is ofthe transmissive type.
 6. The display device of claim 1, wherein a valueobtained by integrating the product of spectral transmission factor ofsaid white light-transmitting filter in the visible range and spectralluminous efficiency with respect to wavelength is less than sum ofvalues obtained by integrating the product of spectral transmissionfactor of each of said non-white light-transmitting filters in thevisible range and spectral luminous efficiency with respect towavelength.
 7. The display device of claim 1, wherein brightness createdby a first gray level represented via said white light-transmittingfilter is lower than brightness created by a first gray levelrepresented via said non-white light-transmitting filters.